1. Field of the Invention
The present invention relates to a device for writing on thermographic material. This device includes a heater for pre-heating the thermographic material to a temperature below a writing temperature required for writing on the thermographic material, and a writing instrument for writing on the thermographic material according to a predefined information signal s(t), wherein the writing instrument is spaced apart from the thermographic material.
2. Description of the Related Art
A device of this type is described in EP 0 734 870 A2. This known device preheats a thermographic material using a heater in form of a rotatably supported heater drum to a temperature below a writing temperature of the thermographic material. The pre-heating step, however, is designed to not induce writing on the thermographic material. The light beam of a single laser is projected on the thermographic material using an optical device. The laser is modulated with an information signal. The thermographic material includes a layer for converting radiation energy into thermal energy. When the modulated laser beam impinges on this layer, thermal energy corresponding to the information signal is produced in the thermographic material. The thermal energy is superimposed on the thermal energy produced by the pre-heating step, thereby exceeding the writing temperature of the thermographic material. The thermographic material is thereby blackened with a density variation corresponding to the information signal modulating the laser. The blackening of the thermographic material occurs row-by-row, with the pixels of the row being blackened consecutively. The optical device projecting the laser beam on the thermographic material has a polygon mirror which rotates at a very high rotation speed. The laser beam is reflected by the polygon mirror, so that the entire row of the thermographic material can be blackened by the laser beam. The laser beam is aimed from one end of the row of the thermographic material to the other end. To blacken the next row of the thermographic material, the heater drum and thus also the thermographic material are rotated by another row width.
The known device requires a complex mirror and lens arrangement for focusing and steering the laser beam to write the entire row. Since the required optical path is quite long, it may not be possible to accurately steer the laser beam. Moreover, the polygon mirror must be adjusted and supported very precisely and also has to rotate at an extremely high rotation speed so that the material can be blackened in a sufficiently short time.
EP 0 424 175 A2 describes a device for exposing photosensitive material.
This device has a plurality of individually addressable light emitting diodes (LED) arranged side-by-side, so that the light sensitive material can be exposed pixel-by-pixel. A lens arrangement is placed between the LED's and the photosensitive material to focus the light beams emitted by the LED's. The known exposure device can eliminate intensity variations between adjacent pixels by transmitting an identical light energy to the pixels in a region of the light sensitive material. LED's emit light at very low energy and can therefore not be used to write on thermographic material.
It is therefore the object of the present invention to provide a compact device based on conventional devices, which makes it possible to write on thermographic material in a simple manner.